Method for performing handover, user equipment, base station, and radio communication system

ABSTRACT

According to embodiments, a base station includes a radio communication unit configured to establish communication with a mobile communication terminal using a plurality of component carriers. The base station further includes a control unit configured to transmit a command to the mobile communication terminal to reduce the plurality of component carriers to one prior to handover of communication between the mobile communication terminal and the base station to another base station.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/556,730, filed Nov. 14, 2005, which is a National Stage ofPCT/JP04/06181, filed May 13, 2004, and claims the benefit of priorityunder 35 U.S.C. §119 of Japanese Application No. 2003-13888, filed May16, 2003. The entire contents of which are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a method for performing a handover, auser equipment, a base station, and a radio communication system.

BACKGROUND ART

In Long Term Evolution-Advanced (LTE-A), which is the next-generationcellular communication standard that is discussed in Third GenerationPartnership Project (3GPP), introduction of technology called carrieraggregation (CA) has been studied. The carrier aggregation is technologythat forms a communication channel between a user equipment (UE) and abase station (BS, or evolved Node B (eNB)) by aggregating a plurality offrequency bands that are supported in LTE, for example, and therebyimproves communication throughput. Each frequency band included in onecommunication channel by the carrier aggregation is called a componentcarrier (CC). The bandwidths of frequency bands that are available inLTE are 1.4 MHz, 3.0 MHz, 5.0 MHz, 10 MHz, 15 MHz, and 20 MHz.Accordingly, if five bands of 20 MHz are aggregated as componentcarriers, a communication channel of 100 MHz in total can be formed.

Component carriers that are included in one communication channel in thecarrier aggregation are not necessarily contiguous to one another in thefrequency direction. The mode in which component carriers are arrangedcontiguous to one another in the frequency direction is called acontiguous mode. On the other hand, the mode in which component carriersare arranged not contiguous to one another is called a non-contiguousmode.

Further, in the carrier aggregation, the number of component carriers inan uplink and the number of component carriers in a downlink are notnecessarily equal. The mode in which the number of component carriers inan uplink and the number of component carriers in a downlink are equalis called a symmetric mode. On the other hand, the mode in which thenumber of component carriers in an uplink and the number of componentcarriers in a downlink are not equal is called an asymmetric mode. Forexample, in the case of using two component carriers in an uplink andthree component carriers in a downlink, it is asymmetric carrieraggregation.

Further, in LTE, any one of frequency division duplex (FDD) and timedivision duplex (TDD) can be used as duplex operation. Because thedirection of a link (uplink or downlink) of each component carrier doesnot change in time in FDD, FDD is better suited to the carrieraggregation compared to TDD.

A handover, which is a basic technique for achieving the mobility of auser equipment in the cellular communication standard, is one ofimportant subjects in LTE-A. In LTE, a user equipment measures acommunication quality over a channel with a serving base station (acurrently connected base station) and communication qualities withperipheral base stations and transmits a measurement report containingmeasurements to the serving base station. Receiving the measurementreport, the serving base station determines whether to execute ahandover based on the measurements contained in the report. Then, if itis determined that a handover is to be executed, a handover is carriedout among a source base station (the serving base station before ahandover), the user equipment, and a target base station (a serving basestation after a handover) in accordance with a prescribed procedure(e.g. cf. Patent Literature 1 below)

CITATION LIST Patent Literature

[PTL 1]

Japanese Unexamined Patent Application Publication No. 2009-232293

SUMMARY OF INVENTION Technical Problem

However, no case has been reported where active consideration is givento how to carry out a handover procedure in a radio communicationinvolving the carrier aggregation.

In the above-described existing handover procedure, processing such asrequest for a handover, confirmation of the request, issue of a handovercommand or random access to a target base station is performed based onthe assumption that one communication channel is composed of onecomponent carrier. In a radio communication involving the carrieraggregation also, it is desirable to perform such processing in the samemanner as the existing procedure in order not to cause a significantimpact on a system or a device which is widely used today.

In light of the foregoing, it is desirable to provide a novel andimproved method for performing a handover, user equipment, base station,and radio communication system that can carry out a handover during aradio communication involving the carrier aggregation without causing asignificant impact on an existing system or device.

Solution to Problem

According to some embodiments, a base station includes a radiocommunication unit configured to establish communication with a mobilecommunication terminal using a plurality of component carriers. The basestation further includes a control unit configured to transmit a commandto the mobile communication terminal to reduce the plurality ofcomponent carriers to one prior to handover of communication between themobile communication terminal and the base station to another basestation.

According to some embodiments, a mobile communication terminal includesa radio communication unit configured to establish communication with abase station using a plurality of component carriers. The mobilecommunication terminal further includes a control unit configured toreceive a command from the base station to reduce the plurality ofcomponent carriers to one prior to handover of communication between themobile communication terminal and the base station to another basestation.

According to some embodiments, a communication system includes a firstbase station and a mobile communication terminal. The first base stationincludes a radio communication unit configured to establishcommunication with a mobile communication terminal using a plurality ofcomponent carriers. The first base station further includes a firstcontrol unit configured to transmit a command to the mobilecommunication terminal to reduce the plurality of component carriers toone prior to handover of communication between the mobile communicationterminal and the base station to a second base station. The mobilecommunication terminal includes a second control unit configured toreduce the plurality of component carriers to one upon reception of thecommand from the first base station.

According to some embodiments, a non-transitory computer readable mediumhaving instructions stored thereon, which when executed by a processorin a base station causes the processor to establish communication with amobile communication terminal using a plurality of component carriers.The instructions further cause the processor to transmit a command tothe mobile communication terminal to reduce the plurality of componentcarriers to one prior to handover of communication between the mobilecommunication terminal and the base station to another base station.

Advantageous Effects of Invention

As described above, the method for performing a handover, the userequipment, the base station, and the radio communication systemaccording to the embodiments of the present invention can carry out ahandover during a radio communication involving the carrier aggregationwithout causing a significant impact on an existing system or device.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sequence chart to describe a flow of a typical handoverprocedure.

FIG. 2 is an explanatory view to describe an example of a structure of acommunication resource.

FIG. 3 is a schematic view showing an outline of a radio communicationsystem according to an embodiment.

FIG. 4 is a block diagram showing an example of a configuration of auser equipment according to a first embodiment.

FIG. 5 is a block diagram showing an example of a detailed configurationof a radio communication unit according to the first embodiment.

FIG. 6 is a block diagram showing an example of a configuration of abase station according to the first embodiment.

FIG. 7 is a sequence chart showing an example of a flow along a firstscenario of a handover procedure according to the first embodiment.

FIG. 8 is a sequence chart showing an example of a flow along a secondscenario of a handover procedure according to the first embodiment.

FIG. 9 is a block diagram showing an example of a configuration of auser equipment according to a second embodiment.

FIG. 10 is a block diagram showing an example of a configuration of abase station according to the second embodiment.

FIG. 11 is a sequence chart showing an example of a flow along a firstscenario of a handover procedure according to the second embodiment.

FIG. 12 is a sequence chart showing an example of a flow along a secondscenario of a handover procedure according to the second embodiment.

FIG. 13 is a block diagram showing an example of a configuration of auser equipment according to a third embodiment.

FIG. 14 is a block diagram showing an example of a configuration of abase station according to the third embodiment.

FIG. 15 is a sequence chart showing an example of a flow of a handoverprocedure according to the third embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the appended drawings. Note that,in this specification and the appended drawings, structural elementsthat have substantially the same function and structure are denoted withthe same reference numerals, and repeated explanation of thesestructural elements is omitted.

Preferred embodiments of the present invention will be describedhereinafter in the following order.

1. Description of Related Art

-   -   1-1. Handover Procedure    -   1-2. Structure of Communication Resource

2. Outline of Radio Communication System

3. Description of First Embodiment

-   -   3-1. Exemplary Configuration of User Equipment    -   3-2. Exemplary Configuration of Base Station    -   3-3. Flow of Process    -   3-4. Summary of First Embodiment

4. Description of Second Embodiment

-   -   4-1. Exemplary Configuration of User Equipment    -   4-2. Exemplary Configuration of Base Station    -   4-3. Flow of Process    -   4-4. Summary of Second Embodiment

5. Description of Third Embodiment

-   -   5-1. Exemplary Configuration of User Equipment    -   5-2. Exemplary Configuration of Base Station    -   5-3. Flow of Process    -   5-4. Summary of Third Embodiment        <1. Description of Related Art>        (1-1. Handover Procedure)

A technique related to the present invention is described hereinafterwith reference to FIGS. 1 and 2. FIG. 1 shows a flow of a handoverprocedure in conformity with LTE in a radio communication not involvingthe carrier aggregation as an example of a typical handover procedure.In this example, a user equipment (UE), a source base station (sourceeNB), a target base station (target eNB), and a mobility managemententity (MME) are involved in the handover procedure.

As a preliminary step toward a handover, the user equipment firstreports the channel quality of a communication channel between the userequipment and the source base station to the source base station (stepS2). The channel quality may be reported on a regular basis or when thechannel quality falls below a predetermined reference value. The userequipment can measure the channel quality of the communication channelwith the source base station by receiving a reference signal containedin a downlink channel from the source base station.

Then, the source base station determines the needs of measurement basedon the quality report received from the user equipment and, ifmeasurement is necessary, allocates measurement gaps to the userequipment (step S4).

Then, the user equipment searches for a downlink channel from aperipheral base station (i.e. performs cell search) during the periodsof the allocated measurement gaps (step S12). Note that the userequipment can recognize a peripheral base station to search according toa list that is provided in advance from the source base station.

When the user equipment acquires synchronization with a downlinkchannel, the user equipment performs measurement by using a referencesignal contained in the downlink channel (step S14). During this period,the source base station restricts an allocation of data communicationrelated to the user equipment so as to avoid occurrence of datatransmission by the user equipment.

Upon completion of the measurement, the user equipment transmits ameasurement report containing measurements to the source base station(step S22). The measurements contained in the measurement report may bethe average value or the central value of measured values over aplurality of times of measurement or the like. Further, the measurementsmay contain data about a plurality of frequency bands.

Receiving the measurement report, the source base station determineswhether or not to execute a handover based on the contents of themeasurement report. For example, when the channel quality of anotherbase station in the periphery is higher than the channel quality of thesource base station by a predetermined threshold or greater, it can bedetermined that a handover is necessary. In this case, the source basestation determines to carry out a handover procedure with the relevantanother base station as a target base station, and transmits a handoverrequest message to the target base station (step S24).

Receiving the handover request message, the target base stationdetermines whether it is possible to accept the user equipment accordingto the availability of a communication service offered by itself or thelike. When it is possible to accept the user equipment, the target basestation transmits a handover request confirm message to the source basestation (step S26).

Receiving the handover request confirm message, the source base stationtransmits a handover command to the user equipment (step S28). Then, theuser equipment acquires synchronization with the downlink channel of thetarget base station (step S32). After that, the user equipment makes arandom access to the target base station by using a random accesschannel in a given time slot (step S34). During this period, the sourcebase station forwards data addressed to the user equipment to the targetbase station (step S36). Then, after success in the random access, theuser equipment transmits a handover complete message to the target basestation (step S42).

Receiving the handover complete message, the target base stationrequests the MME to perform route update for the user equipment (stepS44). Upon updating the route of user data by the MME, the userequipment becomes able to communicate with another device through a newbase station (i.e. the target base station). Then, the target basestation transmits acknowledgement to the user equipment (step S46). Aseries of handover procedure thereby ends.

(1-2. Structure of Communication Resource)

FIG. 2 shows a structure of a communication resource in LTE as anexample of a structure of a communication resource to which the presentinvention is applicable. Referring to FIG. 2, the communication resourcein LTE is segmented in the time direction into radio frames each havinga length of 10 msec. One radio frame includes ten sub-frames, and onesub-frame is made up of two 0.5 msec slots. In LTE, the sub-frame is oneunit of an allocation of a communication resource to each user equipmentin the time direction. Such one unit is called a resource block. Oneresource block includes twelve sub-carriers in the frequency direction.Specifically, one resource block has a size of 1 msec with 12sub-carriers in the time-frequency domain. Throughput of datacommunication increases as a larger number of resource blocks areallocated for data communication on condition of the same bandwidth andtime length. Further, in such a structure of a communication resource, apart of radio frame with a given frequency band is reserved as a randomaccess channel. The random access channel can be used for an access to abase station by a user equipment that has changed from an idle mode toan active mode or an initial access to a target base station in ahandover procedure, for example.

<2. Outline of Radio Communication System>

FIG. 3 is a schematic view showing an outline of a radio communicationsystem 1 according to an embodiment of the present invention. Referringto FIG. 3, the radio communication system 1 includes a user equipment100, a base station 200 a and a base station 200 b. It is assumed thatthe base station 200 a is a serving base station for the user equipment100.

The user equipment 100 is located inside a cell 202 a where a radiocommunication service is provided by the base station 200 a. The userequipment 100 can perform a data communication with another userequipment (not shown) via the base station 200 a over a communicationchannel formed by aggregating a plurality of component carriers (i.e. bycarrier aggregation). However, because the distance between the userequipment 100 and the base station 200 a is not short, there is apossibility that a handover is required for the user equipment 100.Further, the user equipment 100 is located inside a cell 202 b where aradio communication service is provided by the base station 200 b.Therefore, the base station 200 b can be a candidate for a target basestation for a handover of the user equipment 100.

The base station 200 a can communicate with the base station 200 bthrough a backhaul link (e.g. X2 interface). Various kinds of messagesin the handover procedure as described with reference to FIG. 1,scheduling information related to the user equipment belonging to eachcell or the like, for example, can be transmitted and received betweenthe base station 200 a and the base station 200 b. Further, the basestation 200 a and the base station 200 b can communicate with the MME,which is an upper node, through S1 interface, for example.

It is now assumed that the necessity of a handover to the base station200 b arises while the user equipment 100 performs a radio communicationinvolving the carrier aggregation with the base station 200 a. In thiscase, as a way to carry out a handover among the user equipment 100, thebase station 200 a and the base station 200 b without largely alteringthe existing handover procedure described with reference to FIG. 1, thenumber of component carriers constituent of a communication channel maybe shrunk temporarily. Shrinking the number of component carriers meansreducing the number of component carriers constituent of onecommunication channel by the carrier aggregation technology. Forexample, if the number of component carriers is temporarily shrunk toone, a handover can be carried out according to the same procedure asthe existing handover procedure. However, the reduction of the number ofcomponent carriers can cause a temporary decrease in throughput. Thedecrease in throughput raises the possibility of the occurrence ofbuffer under flow at the receiving end of data or buffer over flow atthe transmitting end of data, which can lead to a failure of acommunication service such as contents delivery. Therefore, in the caseof temporarily shrinking the number of component carriers, it iseffective to avoid or reduce the above-described possibility due to adecrease in throughput as much as possible as in the first to thirdembodiments of the present invention which are described in detail inthe following sections.

It should be noted that, when there is no particular need to distinguishbetween the base station 200 a and the base station 200 b in thefollowing description of the specification, they are collectivelyreferred to as a base station 200 by omitting the alphabetical letter atthe end of the reference symbol. The same applies to the other elements.

<3. Description of First Embodiment>

A first embodiment of the present invention is described hereinafterwith reference to FIGS. 4 to 8.

(3-1. Exemplary Configuration of User Equipment)

FIG. 4 is a block diagram showing an example of a configuration of theuser equipment 100 according to the embodiment. Referring to FIG. 4, theuser equipment 100 includes a radio communication unit 110, a signalprocessing unit 150, a buffer 152, a control unit 160, and a measurementunit 170.

(Radio Communication Unit)

The radio communication unit 110 performs a radio communication with thebase station 200 over a communication channel formed by aggregating aplurality of component carriers with use of the carrier aggregationtechnology.

FIG. 5 is a block diagram showing an example of a more detailedconfiguration of the radio communication unit 110. Referring to FIG. 5,the radio communication unit 110 includes an antenna 112, a switch 114,a low noise amplifier (LNA) 120, a plurality of down-converters 122 a to122 c, a plurality of filters 124 a to 124 c, a plurality ofanalogue-to-digital converters (ADCs) 126 a to 126 c, a demodulationunit 128, a modulation unit 130, a plurality of digital-to-analogueconverters (DACs) 132 a to 132 c, a plurality of filters 134 a to 134 c,a plurality of up-converters 136 a to 136 c, a combiner 138, and a poweramplifier (PA) 140.

The antenna 112 receives a radio signal transmitted from the basestation 200 and outputs the received signal to the LNA 120 through theswitch 114. The LNA 120 amplifies the received signal. Thedown-converter 122 a and the filter 124 a separate a baseband signal ofthe first component carrier (CC1) from the received signal amplified bythe LNA 120. Then, the separated baseband signal is converted to adigital signal by the ADC 126 a and output to the demodulation unit 128.Likewise, the down-converter 122 b and the filter 124 b separate abaseband signal of the second component carrier (CC2) from the receivedsignal amplified by the LNA 120. Then, the separated baseband signal isconverted to a digital signal by the ADC 126 b and output to thedemodulation unit 128. Further, the down-converter 122 c and the filter124 c separate a baseband signal of the third component carrier (CC3)from the received signal amplified by the LNA 120. Then, the separatedbaseband signal is converted to a digital signal by the ADC 126 c andoutput to the demodulation unit 128. After that, the demodulation unit128 generates a data signal by demodulating the baseband signals of therespective component carriers and outputs the data signal to the signalprocessing unit 150.

Further, when a data signal is input from the signal processing unit150, the modulation unit 130 modulates the data signal and generatesbaseband signals of the respective component carriers. Among thosebaseband signals, the baseband signal of the first component carrier(CC1) is converted to an analog signal by the DAC 132 a. Then, afrequency component corresponding to the first component carrier in atransmission signal is generated from the analog signal by the filter134 a and the up-converter 136 a. Likewise, the baseband signal of thesecond component carrier (CC2) is converted to an analog signal by theDAC 132 b. Then, a frequency component corresponding to the secondcomponent carrier in the transmission signal is generated from theanalog signal by the filter 134 b and the up-converter 136 b. Further,the baseband signal of the third component carrier (CC3) is converted toan analog signal by the DAC 132 c. Then, a frequency componentcorresponding to the third component carrier in the transmission signalis generated from the analog signal by the filter 134 c and theup-converter 136 c. After that, the generated frequency componentscorresponding to the three component carriers are combined by thecombiner 138, and the transmission signal is formed. The PA 140amplifiers the transmission signal and outputs the transmission signalto the antenna 112 through the switch 114. Then, the antenna 112transmits the transmission signal as a radio signal to the base station200.

Although the case where the radio communication unit 110 handles threecomponent carriers is described in FIG. 5, the number of componentcarriers handled by the radio communication unit 110 may be two, or fouror more.

Further, instead of processing the signals of the respective componentcarriers in the analog region as in the example of FIG. 5, the radiocommunication unit 110 may process the signals of the respectivecomponent carriers in the digital region. In the latter case, at thetime of reception, a digital signal converted by one ADC is separatedinto the signals of the respective component carriers by a digitalfilter. Further, at the time of transmission, after digital signals ofthe respective component carriers are frequency-converted and combined,the signal is converted into an analog signal by one DAC. The load ofthe ADC and the DAC is generally smaller when processing the signals ofthe respective component carriers in the analog region. On the otherhand, when processing the signals of the respective component carriersin the digital region, a sampling frequency for AD/DA conversion ishigher, and the load of the ADC and the DAC can thereby increase.

(Signal Processing Unit)

Referring back to FIG. 4, an example of a configuration of the userequipment 100 is further described below.

The signal processing unit 150 performs signal processing such asdeinterleaving, decoding or error correction on the demodulated datasignal that is input from the radio communication unit 110. Then, thesignal processing unit 150 outputs the processed data signal to an upperlayer. Note that the signal processing unit 150 performs buffer controlof data signals by using the buffer 152. Specifically, the signalprocessing unit 150 stores processed data signals into the buffer 152once and then outputs the data signal to an upper layer, following thefirst-in first-out (FIFO) rule, for example. Thus, when the data signalsare input at a rate exceeding a normal data rate of a communicationservice, the amount of data accumulated in the buffer 152 increases.Further, when the input rate of the data signals decreases, the amountof data accumulated in the buffer 152 decreases. Further, the signalprocessing unit 150 performs signal processing such as encoding orinterleaving on the data signal that is input from the upper layer. Inthis case also, the signal processing unit 150 may perform buffercontrol of data signals by using the buffer 152. Then, the signalprocessing unit 150 outputs the processed data signals to the radiocommunication unit 110.

(Buffer)

The buffer 152 temporarily accumulates the data signals that are inputfrom the signal processing unit 150 by using a storage medium such as ahard disk or semiconductor memory. The data signals are read followingthe first-in first-out rule, for example, and processed by the signalprocessing unit 150.

(Control Unit)

According to some embodiments, the control unit is further configured torestart use of the plurality of component carriers after completion ofthe handover. In further embodiments, the control unit is furtherconfigured to receive a command from the base station to increase datathroughput of the communication between the mobile communicationterminal and the base station prior to reception of the command toreduce the plurality of component carriers to one. In additionalembodiments, the command from the base station to increase the datathroughput instructs the control unit to increase the plurality ofcomponent carriers used between the base station and mobilecommunication terminal.

The control unit 160 controls the overall functions of the userequipment 100 by using a processing device such as a central processingunit (CPU) or a digital signal processor (DSP). For example, the controlunit 160 controls the timing of data communication by the radiocommunication unit 110 according to scheduling information that isreceived from the base station 200 by the radio communication unit 110.Further, the control unit 160 increases or decreases the number ofcomponent carriers constituent of the communication channel with thebase station 200 according to a command from the base station 200. Forexample, when an expansion (increase) command of the number of componentcarriers is received from the base station 200, the control unit 160increases the number of component carriers. Further, when a shrinkcommand of the number of component carriers is received from the basestation 200, the control unit 160 decreases the number of componentcarriers. Besides, the control unit 160 controls the user equipment 100to operate in the same manner as the user equipment in the handoverprocedure which is descried with reference to FIG. 1.

(Measurement Unit)

The measurement unit 170 measures the channel quality for each of thecomponent carriers by using a reference signal from the base station 200according to control from the control unit 160, for example. Further,the measurement unit 170 executes measurement for a handover withrespect to each of the component carriers by using the measurement gapswhich are allocated by the base station 200. A result of the measurementexecuted by the measurement unit 170 is converted to a predeterminedformat for a measurement report by the control unit 160 and transmittedto the base station 200 through the radio communication unit 110. Afterthat, the base station 200 determines, based on the measurement report,whether a handover should be executed or not for the user equipment 100.

(3-2. Exemplary Configuration of Base Station)

FIG. 6 is a block diagram showing an example of a configuration of thebase station 200 according to the embodiment. Referring to FIG. 6, thebase station 200 includes a radio communication unit 210, an interfaceunit 250, a component carrier (CC) management unit 260, and a controlunit 280.

(Radio Communication Unit)

A specific configuration of the radio communication unit 210 may besimilar to the configuration of the radio communication unit 110 of theuser equipment 100 which is described above with reference to FIG. 5,although the number of component carriers to be supported, therequirements of processing performance or the like are different. Theradio communication unit 210 performs a radio communication with theuser equipment over a communication channel which is formed byaggregating a plurality of component carriers with use of the carrieraggregation technology.

(Interface Unit)

The interface unit 250 mediates a communication between the radiocommunication unit 210 or the control unit 280 and an upper node throughthe S1 interface illustrated in FIG. 3, for example. Further, theinterface unit 250 mediates a communication between the radiocommunication unit 210 or the control unit 280 and another base stationthrough the X2 interface illustrated in FIG. 3, for example.

(CC Management Unit)

The CC management unit 260 holds data that indicates which componentcarrier each user equipment is using for communication with respect toeach of the user equipments belonging to the cell of the base station200. Such data can be updated by the control unit 280 when an additionaluser equipment joins the cell of the base station 200 or when theconnected user equipment changes its component carriers. Thus, thecontrol unit 280 can recognize which component carrier the userequipment 100 is using by referring to the data held by the CCmanagement unit 260.

(Control Unit)

According to some embodiments, the control unit is further configured toincrease data throughput of the communication between the mobilecommunication terminal and the base station prior to transmitting thecommand to reduce the plurality of component carriers to one. In furtherembodiments, the control unit is further configured to transmit acommand to the mobile communication terminal to increase the pluralityof component carriers used between the base station and mobilecommunication terminal to increase the data throughput. In additionalembodiments, the control unit is further configured to increase one ormore resource blocks to be allocated to the mobile communicationterminal in at least one component carrier from the plurality ofcomponent carriers to increase the data throughput.

The control unit 280 controls the overall functions of the base station200 by using a processing device such as a CPU or a DSP. For example,the control unit 280 allocates communication resources for datacommunication to the user equipment 100 and other user equipments andthen delivers scheduling information over a broadcast channel in a givensub-frame.

Further, in this embodiment, in the case where the base station 200 is asource base station, when a handover request is confirmed by a targetbase station, the control unit 280 temporarily increases the amount ofcommunication resources to be allocated to the user equipment 100. Thecontrol unit 280 may increase the above-described amount ofcommunication resources by increasing the number of component carriersconstituent of the communication channel with the user equipment 100,for example. The number of component carriers can be increased bytransmitting an expansion command of the number of component carriers tothe user equipment 100, for example. Alternatively, the control unit 280may increase the above-described amount of communication resources byincreasing the number of resource blocks to be allocated to the userequipment 100 in at least one component carrier. After the control unit280 transmits/receives data to/from the user equipment 100 at a higherrate than usual by using the communication resources which are increasedin this manner, before transmitting a handover command to the userequipment 100, the control unit 280 transmits a shrink command of thenumber of component carriers to the user equipment 100. The number ofcomponent carriers constituent of the communication channel between theuser equipment 100 and the base station 200 is thereby shrunk to one.Then, the control unit 280 transmits a handover command for onecomponent carrier after the shrinkage to the user equipment 100.

Further, in the case where the base station 200 is a target basestation, when a handover by the user equipment 100 is completed, thecontrol unit 280 restarts the radio communication involving the carrieraggregation by the user equipment 100 in response to a request from theuser equipment 100. Besides, the control unit 280 controls the basestation 200 to operate in the same manner as the source base station orthe target base station in the handover procedure which is descried withreference to FIG. 1.

(3-3. Flow of Process)

Two scenarios of a handover procedure according to the embodiment aredescribed hereinbelow. Note that, in the following scenarios, it isassumed that a handover procedure is performed among the user equipment100, the base station 200 a serving as a source base station, and thebase station 200 b serving as a target base station. Further, for theprocedure up to measurement in the user equipment (steps S2 to S14) inthe typical handover procedure illustrated in FIG. 1, explanation isomitted because there is no significant difference.

FIG. 7 is a sequence chart showing an example of a flow along the firstscenario of a handover procedure according to the embodiment. Referringto FIG. 7, the user equipment 100 first transmits a measurement reportfor a plurality of component carriers constituent of a communicationchannel to the base station 200 a (step S122). Receiving the measurementreport, the base station 200 a determines the necessity of a handoverbased on the measurement report. For example, when a channel qualitybetween the user equipment 100 and the base station 200 b is better thana channel quality between the user equipment 100 and the base station200 a by a predetermined threshold or greater in any component carrier,it can be determined that a handover is necessary. In this case, thebase station 200 a transmits a handover request message to the basestation 200 b (step S124). Receiving the handover request message, thebase station 200 b determines whether it is possible to accept the userequipment 100 according to the availability of a communication serviceoffered by itself or the like. When the base station 200 b determinesthat it is possible to accept the user equipment 100, the base station200 b transmits a handover request confirm message to the base station200 a (step S126).

Receiving the handover request confirm message, the base station 200 atransmits an expansion command for increasing the number of componentcarriers to the user equipment 100 (step S130). In response thereto, theuser equipment 100 makes a random access to the base station 200 a for anew component carrier to establish a communication channel with the basestation 200 a and thereby increases the number of component carriers(step S132). After that, the base station 200 a transmits data to theuser equipment 100 at a rate exceeding a normal data rate over theexpanded communication channel (step S134). The transmitted data istemporarily accumulated in the buffer 152 of the user equipment 100.

Then, the base station 200 a transmits a shrink command for decreasingthe number of component carriers to the user equipment 100 (step S136).In response thereto, the user equipment 100 shrinks the number ofcomponent carriers constituent of the communication channel with thebase station 200 a to one (step S140). The base station 200 a thentransmits a handover command for one component carrier after theshrinkage to the user equipment 100 (step S142).

Receiving the handover command, the user equipment 100 acquiressynchronization with the downlink channel of the base station 200 b(step S152). Then, the user equipment 100 makes a random access to thebase station 200 b by using a random access channel in a given time slotof the synchronized downlink channel (step S154). During this period,the base station 200 a forwards data addressed to the user equipment 100to the base station 200 b (step S156). Then, after success in the randomaccess, the user equipment 100 transmits a handover complete message tothe base station 200 b (step S162). Receiving the handover completemessage, the base station 200 b requests the MME to perform route updatefor the user equipment 100 (step S164). Upon updating the route of userdata by the MME, the user equipment 100 becomes able to communicate withanother device through a new base station (i.e. the base station 200 b).Then, the base station 200 b transmits acknowledgement for the handovercomplete message to the user equipment 100 (step S166).

After that, the user equipment 100 restarts the radio communicationinvolving the carrier aggregation with the base station 200 b, which isa new serving base station (step S180). Specifically, the user equipment100 makes a random access to the base station 200 b for an additionalcomponent carrier to establish a communication channel with the basestation 200 b, for example, and thereby increases the number ofcomponent carriers. It is thereby possible in the user equipment 100 touse the communication service again at substantially the same data rateas that before the start of the handover procedure.

FIG. 8 is a sequence chart showing an example of a flow along the secondscenario of a handover procedure according to the embodiment. As isunderstood from comparison between FIG. 7 and FIG. 8, in the secondscenario, the steps S130 and S132 in the first scenario are replaced bystep S131.

In the second scenario, the base station 200 a which has received thehandover request confirm message first increases the number of resourceblocks to be allocated to the user equipment 100 in at least onecomponent carrier. The amount of communication resources available forthe user equipment 100 thereby increases temporarily (step S131). Afterthat, the base station 200 a transmits data to the user equipment 100 ata rate exceeding a normal data rate by using the temporarily increasedcommunication resources (step S134). The transmitted data is temporarilyaccumulated in the buffer 152 of the user equipment 100. Then, in thesame procedure as in the first scenario, after the number of componentcarriers constituent of the communication channel between the userequipment 100 and the base station 200 a is shrunk to one, a handoverfrom the base station 200 a to the base station 200 b is carried out.The radio communication involving the carrier aggregation is thenrestarted between the user equipment 100 and the base station 200 b.

(3-4. Summary of First Embodiment)

The first embodiment of the present invention is described above withreference to FIGS. 4 to 8. According to the embodiment, before ahandover command is transmitted from the source base station to the userequipment, the number of component carriers constituent of thecommunication channel between the user equipment and the source basestation is shrunk to one. It is thereby possible to carry out theprocedure from transmission of a handover command to completion of ahandover in the same procedure as the existing handover procedure.

Further, according to the embodiment, before the number of componentcarriers is shrunk to one, the amount of communication resourcesallocated to the user equipment is temporarily increased, and data istransmitted to the user equipment at a rate exceeding a normal data rateby using the communication resources. It is thereby possible to reducethe possibility of the occurrence of a failure caused by buffer underflow in the user equipment during the period until the user equipmentrestarts carrier aggregation with the target base station. Further, atemporary increase in the amount of communication resources can be madeby an increase in the number of component carriers or an increase in thenumber of resource blocks. Because the increase in the number ofcomponent carriers and the increase in the number of resource blocks canbe made by applying the existing scheme, the impact of such processingon the whole system is small. Further, in this embodiment, because achange in the number of component carriers is made under control of thebase station, the impact on the user equipment, particularly, can beminimized.

Note that, in the step S134 of FIGS. 7 and 8, data may be transmittedfrom the user equipment to the base station, rather than transmittedfrom the base station to the user equipment at a rate exceeding a normaldata rate by using the temporarily increased communication resources. Inthis case, in the user equipment that transmits contents to anotherequipment, a large amount of content data is transmitted in advancebefore the shrinkage of the number of component carriers, for example,thereby avoiding the occurrence of buffer over flow during the handoverprocedure.

<4. Description of Second Embodiment>

A second embodiment of the present invention is described hereinafterwith reference to FIGS. 9 to 12.

(4-1. Exemplary Configuration of User Equipment)

FIG. 9 is a block diagram showing an example of a configuration of auser equipment 300 according to the embodiment. Referring to FIG. 9, theuser equipment 300 includes a radio communication unit 110, a signalprocessing unit 150, a buffer 152, a control unit 360, and a measurementunit 170.

(Control Unit)

The control unit 160 controls the overall functions of the userequipment 300 by using a processing device such as a CPU or a DSP. Forexample, the control unit 360 controls the timing of data communicationby the radio communication unit 110 according to scheduling informationthat is received from a base station 400 by the radio communication unit110. Further, as the preliminary step toward a handover, the controlunit 360 shrinks the number of component carriers constituent of acommunication channel with the base station 400 to one. The control unit360 may shrink the number of component carriers before transmitting ameasurement report to the base station 400 when a result of themeasurement by the measurement unit 170 indicates that a handover shouldbe started, for example. In this embodiment, the control unit 360transmits a shrink request to the base station 400 and, after the shrinkrequest is confirmed by the base station 400, the control unit 360shrinks the number of component carriers to one. Further, beforeshrinking the number of component carriers to one, the control unit 360temporarily increases the amount of communication resources availablefor the user equipment 300. The control unit 360 may temporarilyincrease the amount of communication resources available for the userequipment 300 by increasing the number of component carriers constituentof the communication channel with the base station 400, for example.Alternatively, the base station 400 which has received the shrinkrequest may temporarily increase the number of resource blocks to beallocated to the user equipment 300 in at least one component carrier.After the control unit 360 transmits or receives data at a higher datarate than usual by using the communication resources which are increasedin this manner and further shrinks the number of component carriers toone, the control unit 360 transmits a measurement report to the basestation 400. After that, the control unit 360 controls the userequipment 300 to operate in the same manner as the user equipment in thehandover procedure which is descried with reference to FIG. 1.

(4-2. Exemplary Configuration of Base Station]

FIG. 10 is a block diagram showing an example of a configuration of thebase station 400 according to the embodiment. Referring to FIG. 10, thebase station 400 includes a radio communication unit 210, an interfaceunit 250, a CC management unit 260, and a control unit 480.

(Control Unit)

The control unit 480 controls the overall functions of the base station400 by using a processing device such as a CPU or a DSP. For example,the control unit 480 allocates communication resources for datacommunication to the user equipment 300 and other user equipments andthen delivers scheduling information over a broadcast channel in a givensub-frame.

Further, in this embodiment, in the case where the base station 400 is asource base station, the control unit 480 receives the above-describedshrink request from the user equipment 300 through the radiocommunication unit 210. Further, before or after receiving the shrinkrequest, the control unit 480 temporarily increases the amount ofcommunication resources to be allocated to the user equipment 300 inresponse to a request from the user equipment 300. The control unit 480may increase the above-described amount of communication resources byincreasing the number of component carriers constituent of thecommunication channel with the user equipment 300, for example.Alternatively, the control unit 480 may increase the above-describedamount of communication resources by increasing the number of resourceblocks to be allocated to the user equipment 300 in at least onecomponent carrier, for example. After the control unit 480transmits/receives data to/from the user equipment 300 at a higher ratethan usual by using the communication resources which are increased inthis manner, the control unit 480 confirms the shrink request from theuser equipment 300. As a result, the number of component carriersconstituent of the communication channel between the user equipment 300and the base station 400 is shrunk to one. Then, the control unit 480carries out a handover for one component carrier after the shrinkage.

Further, in the case where the base station 400 is a target basestation, when a handover by the user equipment 300 is completed, thecontrol unit 480 restarts the radio communication involving the carrieraggregation by the user equipment 300 in response to a request from theuser equipment 300.

(4-3. Flow of Process)

Two scenarios of a handover procedure according to the embodiment aredescribed hereinbelow. Note that, in the following scenarios, it isassumed that a handover procedure is performed among the user equipment300, the base station 400 a serving as a source base station, and thebase station 400 b serving as a target base station. Further, for theprocedure up to measurement in the user equipment (steps S2 to S14) inthe typical handover procedure illustrated in FIG. 1, explanation isomitted because there is no significant difference.

FIG. 11 is a sequence chart showing an example of a flow along the firstscenario of a handover procedure according to the embodiment. Referringto FIG. 11, the user equipment 300 first makes a random access to thebase station 400 a for a new component carrier to establish acommunication channel with the base station 400 a and thereby increasesthe number of component carriers (step S212). After that, the basestation 400 a transmits data to the user equipment 300 at a rateexceeding a normal data rate over the expanded communication channel(step S214). The transmitted data is temporarily accumulated in thebuffer 152 of the user equipment 300. Note that, in the step S214, datamay be received from the user equipment 300 rather than transmitted tothe user equipment 300.

After the sufficient amount of data to avoid buffer under flow areaccumulated in the buffer 152, for example, the user equipment 300transmits a shrink request to the base station 400 a (step S216). Then,the base station 400 a transmits a confirm message for the shrinkrequest to the user equipment 300 (step S222). The user equipment 300then shrinks the number of component carriers constituent of thecommunication channel with the base station 400 a to one (step S224).

Then, the user equipment 300 transmits a measurement report for onecomponent carrier after the shrinkage to the base station 400 a (stepS232). Receiving the measurement report, the base station 400 atransmits a handover request message to the base station 400 b (stepS234). Receiving the handover request message, the base station 400 bdetermines whether it is possible to accept the user equipment 300according to the availability of a communication service offered byitself or the like. When the base station 400 b determines that it ispossible to accept the user equipment 300, it transmits a handoverrequest confirm message to the base station 400 a (step S236). Receivingthe handover request confirm message, the base station 400 a transmits ahandover command to the user equipment 300 (step S242).

Receiving the handover command, the user equipment 300 continues thehandover procedure for one component carrier after the shrinkage.Specifically, the user equipment 300 first acquires synchronization withthe downlink channel of the base station 400 b (step S252). Then, theuser equipment 300 makes a random access to the base station 400 b byusing a random access channel in a given time slot of the synchronizeddownlink channel (step S254). During this period, the base station 400 aforwards data addressed to the user equipment 300 to the base station400 b (step S256). Then, after success in the random access, the userequipment 300 transmits a handover complete message to the base station400 b (step S262). Receiving the handover complete message, the basestation 400 b requests the MME to perform route update for the userequipment 300 (step S264). Then, the base station 400 b transmitsacknowledgement for the handover complete message to the user equipment300 (step S266).

After that, the user equipment 300 restarts the radio communicationinvolving the carrier aggregation with the base station 400 b, which isa new serving base station (step S280). Specifically, the user equipment300 makes a random access to the base station 400 b for an additionalcomponent carrier to establish a communication channel with the basestation 400 b, for example, and thereby increases the number ofcomponent carriers. It is thereby possible in the user equipment 300 touse the communication service again at substantially the same data rateas that before the start of the handover procedure.

FIG. 12 is a sequence chart showing an example of a flow along thesecond scenario of a handover procedure according to the embodiment. Asis understood from comparison between FIG. 11 and FIG. 12, in the secondscenario, the steps S212 to S222 in the first scenario are replaced bysteps S216 to S222.

In the second scenario, the user equipment 300 which has recognized thata measurement result indicates that a handover should be startedtransmits a shrink request to the base station 400 a (step S216).Receiving the shrink request, the base station 400 a increases thenumber of resource blocks to be allocated to the user equipment 300 inat least one component carrier. The amount of communication resourcesavailable for the user equipment 300 thereby increases temporarily (stepS218). After that, the base station 400 a transmits data to the userequipment 300 at a rate exceeding a normal data rate by using thetemporarily increased communication resources (step S220). Thetransmitted data is temporarily accumulated in the buffer 152 of theuser equipment 300. After that, the base station 400 a transmits aconfirm message for the shrink request to the user equipment 300 (stepS222). Then, in the same procedure as in the first scenario, after thenumber of component carriers constituent of the communication channelbetween the user equipment 300 and the base station 400 a is shrunk toone, a handover from the base station 400 a to the base station 400 b iscarried out. The radio communication involving the carrier aggregationis then restarted between the user equipment 300 and the base station400 b.

(4-4. Summary of Second Embodiment)

The second embodiment of the present invention is described above withreference to FIGS. 9 to 12. According to the embodiment, before ameasurement report is transmitted from the user equipment to the sourcebase station, the number of component carriers constituent of thecommunication channel between the user equipment and the source basestation is shrunk to one. It is thereby possible to carry out theprocedure from transmission of a measurement report to completion of ahandover in the same procedure as the existing handover procedure.

Further, in this embodiment also, before the number of componentcarriers is shrunk to one, the amount of communication resourcesallocated to the user equipment is temporarily increased, and data istransmitted and received at a rate exceeding a normal data rate by usingthe communication resources. It is thereby possible to reduce thepossibility of the occurrence of a failure caused by buffer under flowor buffer over flow in the user equipment during the period until theuser equipment restarts carrier aggregation with the target basestation. Further, in this embodiment, because a change in the number ofcomponent carriers is made in response to a request from the userequipment, the impact on the base station, particularly, can beminimized.

<5. Description of Third Embodiment>

A third embodiment of the present invention is described hereinafterwith reference to FIGS. 13 to 15.

(5-1. Exemplary Configuration of User Equipment)

FIG. 13 is a block diagram showing an example of a configuration of auser equipment 500 according to the embodiment. Referring to FIG. 13,the user equipment 500 includes a radio communication unit 110, a signalprocessing unit 150, a buffer 152, a control unit 560, and a measurementunit 170.

(Control Unit)

The control unit 560 controls the overall functions of the userequipment 500 by using a processing device such as a CPU or a DSP. Forexample, the control unit 560 controls the timing of data communicationby the radio communication unit 110 according to scheduling informationthat is received from a base station 600 by the radio communication unit110. Further, as the preliminary step toward a handover, the controlunit 560 shrinks the number of component carriers constituent of acommunication channel with a source base station to one. Further, aftercompleting a handover for one component carrier after the shrinkage, thecontrol unit 560 temporarily receives an allocation of manycommunication resources from a target base station and performs datacommunication at a rate exceeding a normal data rate.

(5-2. Exemplary Configuration of Base Station)

FIG. 14 is a block diagram showing an example of a configuration of thebase station 600 according to the embodiment. Referring to FIG. 14, thebase station 600 includes a radio communication unit 210, an interfaceunit 250, a CC management unit 260, and a control unit 680.

(Control Unit)

According to some embodiments, the control unit is further configured toadjust a timing of a handover to compensate for a decrease in datathroughput due to the reduction of the plurality of component carriersto one. In further embodiments, the timing of the hand over is decidedbased on a likelihood of success in a random access.

The control unit 680 controls the overall functions of the base station600 by using a processing device such as a CPU or a DSP. For example,the control unit 680 allocates communication resources for datacommunication to the user equipment 500 and other user equipments andthen delivers scheduling information over a broadcast channel in a givensub-frame.

Further, in this embodiment, in the case where the base station 600 is asource base station, when a handover request is confirmed by a targetbase station, the control unit 680 transmits a shrink command to theuser equipment 500 before transmitting a handover command. The number ofcomponent carriers constituent of the communication channel between theuser equipment 500 and the base station 600 is thereby shrunk to one.Then, the control unit 680 transmits a handover command for onecomponent carrier after the shrinkage to the user equipment 500.

Further, in this embodiment, in the case where the base station 600 is atarget base station, when a handover request is received from a sourcebase station, the control unit 680 monitors the availability ofcommunication resources. Then, the control unit 680 adjusts the timingto confirm the handover request from the source base station so that adecrease in throughput due to the shrinkage of the number of componentcarriers is suppressed or compensated.

Specifically, when a random access from the user equipment 500 issuccess, for example, the control unit 680 confirms the handover requestafter waiting until the timing at which a sufficient amount ofcommunication resources can be allocated to the user equipment 500. Thesufficient amount may be the amount of communication resources whichenables an allocation of substantially the same number of componentcarries as the number of component carriers before the handover(shrinkage) to a new communication channel, for example. In this case,by recovering the number of component carriers promptly after thehandover, it is possible to reduce the time during which a throughputdecreases due to the handover. Note that the number of componentcarriers before the handover can be notified from the source basestation to the target base station by advance negotiation or using ahandover request message or another message, for example.

Further, the sufficient amount may be the amount of communicationresources which enables a recovery of the amount of data accumulated inthe buffer of the user equipment at the receiving end of data to theamount not causing buffer under flow, for example. The amount ofcommunication resources may be defined in advance, or notified from asource base station by using a handover request message or the like. Inthis case, after a random access from the user equipment 500 succeeds,the amount of communication resources allocated to the user equipment500 is temporarily increased, thereby compensating a decrease inthroughput that occurs due to the shrinkage of the number of componentcarriers.

Furthermore, the control unit 680 may confirm the handover request afterwaiting until the timing at which it is determined that the possibilityof success in a random access from the user equipment 500 is high. Anexample of the timing when the possibility of success in a random accessis high is timing at which there are not a large number of other userequipments that are trying to make a random access (e.g. another userequipment that has started a handover, a user equipment in the idle modeetc.). In this case, it is possible to prevent a delay of a handovercaused by a failure and retry of a random access and reduce the timeduring which a throughput decreases due to the handover.

(5-3. Flow of Process)

An example of a flow of a handover procedure according to the embodimentis described hereinbelow. Note that, in the following scenario, it isassumed that a handover procedure is performed among the user equipment500, the base station 600 a serving as a source base station, and thebase station 600 b serving as a target base station. Further, for theprocedure up to measurement in the user equipment (steps S2 to S14) inthe typical handover procedure illustrated in FIG. 1, explanation isomitted because there is no significant difference.

FIG. 15 is a sequence chart showing an example of a flow of a handoverprocedure according to the embodiment. Referring to FIG. 15, the userequipment 500 first transmits a measurement report for a plurality ofcomponent carriers constituent of a communication channel to the basestation 600 a (step S322). The base station 600 a which has determinedthat a handover is necessary based on the measurement report transmits ahandover request message to the base station 600 b (step S324).Receiving the handover request message, the base station 600 b adjuststhe timing to confirm the handover request so that a decrease inthroughput due to the shrinkage of the number of component carriers issuppressed or compensated. (step S325). Then, the base station 600 btransmits a handover request confirm message to the base station 600 aat timing (T1) when the subsequent random access is likely to succeed,timing (T2) when more communication resources can be allocated aftercompletion of a handover, or timing (T3) when carrier aggregation can berestarted promptly after completion of a handover, for example (stepS326).

Receiving the handover request confirm message, the base station 600 atransmits a shrink command for decreasing the number of componentcarriers to the user equipment 500 (step S336). After that, the userequipment 500 shrinks the number of component carriers constituent ofthe communication channel with the base station 600 a to one (stepS340). The base station 600 a then transmits a handover command for onecomponent carrier after the shrinkage to the user equipment 500 (stepS342).

Receiving the handover command, the user equipment 500 continues thehandover procedure for one component carrier after the shrinkage.Specifically, the user equipment 500 first acquires synchronization withthe downlink channel of the base station 600 b (step S352). Then, theuser equipment 500 makes a random access to the base station 600 b byusing a random access channel in a given time slot of the synchronizeddownlink channel (step S354). During this period, the base station 600 aforwards data addressed to the user equipment 500 to the base station600 b (step S356). Then, after success in the random access, the userequipment 500 transmits a handover complete message to the base station600 b (step S362). Receiving the handover complete message, the basestation 600 b requests the MME to perform route update for the userequipment 500 (step S364). Then, the base station 600 b transmitsacknowledgement for the handover complete message to the user equipment500 (step S366).

After that, the base station 600 b temporarily allocates a larger numberof resource blocks to the user equipment 500, for example (step S370).Then, the base station 600 b transmits data to the user equipment 500 ata rate exceeding a normal data rate (step S372). The transmitted datarecovers the amount of data which has decreased during the handoverprocedure in the buffer 152 of the user equipment 500. Note that, in thestep S372, data may be received from the user equipment 500, rather thantransmitted to the user equipment 500. After that, the user equipment500 restarts the radio communication involving the carrier aggregationwith the base station 600 b, which is a new serving base station (stepS380).

(5-4. Summary of Third Embodiment)

The third embodiment of the present invention is described above withreference to FIGS. 13 to 15. According to the embodiment, before ahandover command is transmitted from a source base station to a userequipment, the number of component carriers constituent of thecommunication channel between the user equipment and the source basestation is shrunk to one. It is thereby possible to carry out theprocedure from transmission of a handover command to completion of ahandover in the same procedure as the existing handover procedure.

Further, in this embodiment, after success in a random access from auser equipment to a target base station, the amount of communicationresources allocated to the user equipment is temporarily increased inthe target base station. Then, by using the communication resources,data is transmitted/received to/from the user equipment at a rateexceeding a normal data rate. It is thereby possible to compensate adecrease in throughput that occurs due to the shrinkage of the number ofcomponent carriers.

Furthermore, in this embodiment, the timing to confirm the handoverrequest in the target base station is adjusted so as to suppress orcompensate a decrease in throughput due to the shrinkage of the numberof component carriers. It is thereby possible to reduce the possibilitythat a failure occurs in a communication service such as contentsdelivery due to a decrease in throughput.

Although preferred embodiments of the present invention are described indetail above with reference to the appended drawings, the presentinvention is not limited thereto. It should be understood by thoseskilled in the art that various modifications, combinations,sub-combinations and alterations may occur depending on designrequirements and other factors insofar as they are within the scope ofthe appended claims or the equivalents thereof.

[Reference Signs List]

-   1 RADIO COMMUNICATION SYSTEM-   100, 300, 500 USER EQUIPMENT-   110 RADIO COMMUNICATION UNIT (USER EQUIPMENT)-   160, 360, 560 CONTROL UNIT (USER EQUIPMENT)-   200, 400, 600 BASE STATION-   210 RADIO COMMUNICATION UNIT (BASE STATION)-   280, 480, 680 CONTROL UNIT (BASE STATION)

The invention claimed is:
 1. An electronic device comprising: circuitryconfigured to control transmitting or receiving signals via a pluralityof component carriers including a first component carrier; transmit ameasurement report to a base station, wherein the base stationdetermines whether to conduct handover based on the measurement report;and receive, after the base station determines to conduct the handover,and before the electronic device transmits a random access preamble to atarget base station, a deactivation command from the base stationinstructing the electronic device to deactivate the plurality ofcomponent carriers with the exception of the first component carrier. 2.The electronic device of claim 1, wherein the circuitry is furtherconfigured to receive a handover command to change a synchronizingcomponent carrier from the first component carrier to another componentcarrier.
 3. The electronic device of claim 2, wherein the circuitry isfurther configured to control sending the random access preamble to thetarget base station via the another component carrier.
 4. The electronicdevice of claim 3, wherein the circuitry is configured to controlstarting communication with the target base station via only the firstcomponent carrier.
 5. The electronic device of claim 3, wherein thecircuitry is configured to control transmitting the random accesspreamble only after the deactivation command is received.
 6. Acommunication apparatus comprising: an antenna unit; and circuitryconfigured to control transmitting or receiving signals via a pluralityof component carriers including a first component carrier; transmit ameasurement report to a base station, wherein the base stationdetermines whether to conduct handover based on the measurement report;and receive, after the base station determines to conduct the handoverand before the communication apparatus a random access preamble to atarget base station, a deactivation command from the base stationinstructing the electronic device to deactivate the plurality ofcomponent carriers with the exception of the first component carrier. 7.The communication apparatus of claim 6, wherein the circuitry is furtherconfigured to receive a handover command to change a synchronizingcomponent carrier from the first component carrier to another componentcarrier.
 8. The communication apparatus of claim 7, wherein thecircuitry is further configured to control sending the random accesspreamble to the target base station via the another component carrier.9. The communication apparatus of claim 8, wherein the circuitry isconfigured to control starting communication with the target basestation via only the first component carrier.
 10. The communicationapparatus of claim 8, wherein the circuitry is configured to controltransmitting the random access preamble only after the deactivationcommand is received.
 11. A communication method comprising: controllingtransmitting or receiving signals via a plurality of component carriersincluding a first component carrier; transmitting a measurement reportto a base station, wherein the base station determines whether toconduct handover based on the measurement report; and receiving, afterthe base station determines to conduct the handover and beforetransmitting a random access preamble to a target base station,deactivation command from the base station instructing the electronicdevice to deactivate the plurality of component carriers with theexception of the first component carrier.
 12. The method of claim 11,further comprising: receiving a handover command to change asynchronizing component carrier from the first component carrier toanother component carrier.
 13. The method of claim 12, furthercomprising: controlling sending the random access preamble to the targetbase station via the another component carrier.
 14. The method of claim13, further comprising: controlling starting communication with thetarget base station via only the first component carrier.
 15. The methodof claim 13, further comprising: controlling transmitting the randomaccess preamble only after the deactivation command is received.